1. Field of the Invention
The present invention relates to a method of producing polymers in a continuously operated polymerization reactor. In particular, the invention concerns a method of continuously withdrawing solid polymer powder from a fluidized bed polymerization reactor with high throughput. The present invention also concerns an apparatus for continuous withdrawal of polymer powder from a fluidized bed polymerization reactor.
2. Description of Related Art
A number of processes for preparing polymers in a fluidized bed reactor are known in the art. Such processes are described in e.g. EP Patent Specification No. 0 517 868, U.S. Pat. No. 4,543,399 and EP Patent Application No. 0 381 364. The processes are primarily designed for the production of polyethylene but they can also be modified for preparing other polyolefins, such as polypropylene. In the process described in EP Patent Specification No. 0 517 868 the fluidized bed reactor acts as a second reactor in a two-reactor train, in the other above-mentioned processes it is a stand-alone reactor.
Conventional gas phase fluidized bed reactors comprise an elongated reactor body generally having a vertical central axis. The monomers are polymerized in a fluidized bed above a fluidization grid located in the bottom end of the reactor body. A gaseous stream containing monomer, and optionally alpha-olefin comonomer(s), hydrogen and inert gas(es) is introduced to the bottom of the bed through the fluidization grid. The unreacted gas is collected from the top of the bed, cooled and recycled to the bottom of the reactor. The polymer product is withdrawn from the lower part of the reactor above the fluidization grid. Active catalyst is introduced into the bed, either as a fresh catalyst or as polymer particles from a prior polymerization stage, wherein the active catalyst is dispersed.
The polymerization system of a gas phase reactor used for polymerization of α-olefins comprises a fluidized bed consisting of polymer particles containing the active catalyst within, and a gaseous reaction medium. The bed can be maintained in the fluidized state by mechanically mixing or stirring the contents of the reactor and additionally or alternatively by blowing the monomer(s), i.e. the olefin(s), and/or an inert reaction medium (e.g. nitrogen and/or an easily volatile hydrocarbon) into it in a gaseous state. In the case of a fluidized bed process, the velocity of the gas needs to be sufficient to support or fluidize the polymer particles. The monomer(s) and/or an inert reaction medium in liquid state can be introduced into the polymerization system and the polymerization can be carried out while gasifying said monomer(s) and/or reaction medium. The unreacted monomer(s) and/or inert reaction medium can be partly or wholly liquefied and recycled in liquid state into the polymerization system, as disclosed in EP-A1 0 024 933.
As disclosed in a U.S. Pat. No. 4,803,251, sheet formation is occasionally encountered in gas phase reactors. The sheets are particle agglomerates having a thickness of about 6 to 15 mm, length of about 30 to 150 cm and width of 7.5 to 45 cm or more. The sheets consist of fused polymer. According to U.S. Pat. No. 4,803,251, the formation of sheets is associated with static electricity. The possible formation of hot spots, which eventually may lead to chunk formation as disclosed in EP 0 089 691, was also discussed.
Conventionally, the withdrawal of the polymer product from a gas phase reactor is carried out batch-wise. A typical batch discharge process consists of the following steps: When the surface level within the gas phase reactor rises as a result of polymerization, polymer powder is discharged through an on/off valve to an outlet tank. A part of the fluidizing gas entering the tank with powder is recycled by a compressor back to the fluidized bed. The degassed powder is recovered and optionally subjected to further processing.
By means of the batch-wise discharge system described above it is possible to remove not only the polymer powder but also lumps formed in the reactor, because the outlet nozzles of the system have a large diameter, generally much larger than required by the withdrawal of the powder.
There are, however, some serious disadvantages associated with conventional technology. The batchwise discharge is a rather complicated system. It comprises several phases and a number of on/off valves, which typically operate tens of times an hour. There has to be at least two discharge systems, in large plants even more. This feature makes the batch-wise discharge system rather expensive as an investment and also the maintenance is cost-consuming. The complexity of the system also makes it vulnerable; if the discharge system becomes clogged or there is a serious failure in the system, the whole plant has to be shut down.
Further, the batch-wise discharge is inflexible because it is difficult to increase its capacity when the capacity of the whole plant is increased. Instead, a major rise in the production capacity requires the addition of a new expensive discharge system.
As regards the operation of the system, it can be noted that there is a distinct fluctuation in the bed level when a batch of the polymer product is withdrawn from the reactor. This fluctuation influences the monomer concentration and, thus, also other parameters, such as the concentration of hydrogen and comonomers, which all taken together have a strong impact on the quality of the polymer product.
The discontinuous operation of the outlet system also leads to a pulsating operation of the recycle gas compressor. This, again, leads to increased wear.
Some alternative systems for withdrawal of polymer powder from a fluidized bed reactor are also described in the art. Such processes are described in EP Published Patent Applications Nos. 0 006 288 and 0 245 043 and U.S. Pat. No. 4,495,337.
The last-mentioned document discloses a process for bottom drainage of a fluid-bed reactor, in which the reactor is provided not only with a vertical bottom pipe but also with a lateral evacuation pipe located above the distribution plate of the reactor. The side pipe makes it possible to evacuate a part of the polymer formed in the reactor. According to the reference, to empty the reactor, first all the fraction of polymer powder located above the level of the outlet pipe is discharged by means of the lateral pipe, then total drainage of the reactor is carried out via the vertical bottom pipe.
EP A1 0 006 288 and A1 0 245 043 disclose discharge systems which are discontinuously or continuously operated. In EP A1 0 006 288 the discharge is effected via a dipleg, which passes from the top of the reactor to a point near the bottom. The dip leg is connected to an on/off valve, which is opened intermittently. When the valve is open, a mixture of polymer and monomer is passed on to a cyclone in which most of the monomer vapour is separated from the polymer. The monomer can be recycled to the reactor via compressor.
EP A1 0 245 043 teaches an apparatus for degassing and pelleting polyolefins, in which there is an outlet nozzle which is equipped with a lock chamber placed between two valves activated alternately. The mixture of gas and solid particles, withdrawn from the reactor, is treated in a primary degasser for separating the solid phase from a part of the gas phase accompanying the particles. The gases containing monomers are recycled into the reactor by a pipe.
Although the prior art suggests that the above three alternative discharge systems can be not only periodically operated but also continuously, none of them is in fact adapted for such operation. It should be noted that the control valves described in EP A1 0 006 288 and A1 0.245 043 are actually intermittently operated which means that there is no continuous stream of polymer powder through the outlet nozzles. Thus, these systems are merely improved batch discharge systems, which have the same main disadvantages as described above. The discharge system of U.S. Pat. No. 4,495,337 is, on the other hand, suitable for draining of the whole content of the reactor rather than for allowing for continuous withdrawal of polymer powder.
JP-A-58/113208 discloses a process for continuously polymerizing olefins in vapour phase. The level of the fluidized bed is determined by measuring the pressure difference between two specified locations in the bed. The document further indicates that the polymer can be withdrawn either continuously or intermittently. How the suggested continuous withdrawal is made in practice is not disclosed.
EP-A-0 870 539 discloses an apparatus for polymerizing olefins in gas phase. FIG. 2 and column 8, lines 21-55 of the document describe an embodiment, wherein the polymer is withdrawn continuously from the reactor. The document does not refer to the presence of any chunks or sheets among the polymer, and does not suggest how these should be handled when withdrawing polymer from the reactor. It merely discusses how the gas discharge pipe should be installed to prevent it from blocking.